Corrugated film having increased stiffness

ABSTRACT

Corrugated polymeric film can be stiffened in the direction transverse to corrugation lines by creating flattened, i.e., noncorrugated, narrow width lanes in that transverse direction.

United States Patent [191 Davis et a1.

[4 1 Oct. 1, 1974 CORRUGATED FILM HAVING INCREASED STIFFNESS Inventors: Richard Pelle Davis, Wilmington,

Del.; Thomas Edward Dickerhofe, Circleville, Ohio E. I. du Pont de Nemours and Company, Wilmington, Del.

Filed: Jan. 28, 1972 Appl. No.: 221,677

Assignee:

US. Cl 161/134, 156/195, 161/123, 161/133, 161/165, 174/110 SR, 174/110 PM,

Int. Cl. B32b 3/28, H01b 17/62 8] Field of Search l61/123,130,133, 134,

161/165; 174/110 SR, 110 PM; 156/195; 229/35 R References Cited UNITED STATES PATENTS Lane 161/134 Olson 174/120 Thompson et 174/120 Roberts 174/107 Dooley 161/133 Grail 174/107 Primary ExaminerGeorge F. Lesmes Assistant ExaminerChar1es E. Lipsey ABSTRACT Corrugated polymeric film can be stiffened in the direction transverse to corrugation lines by creating flattened, i.e., noncorrugated, narrow width lanes in that transverse direction.

1 Claim, 2 Drawing Figures PATENTEB 1 74 FIG.

FIG.2

CORRUGATED FILM HAVING INCREASED STIFFNESS BACKGROUND OF THE INVENTION This invention relates to corrugated polymeric film having increased stiffness.

Corrugated films are being used increasingly for many applications One use is in the formation of a core wrap for electrical cable as described in Roberts, US. Pat. No. 3,244,799, and assignees application of Everhart et al., Ser. No. 34,812, filed May 5, 1970, now abandoned. In such applications, the corrugated film provides mechanical protection, thermal insulation, dielectric protection and electrical stability for the core. Corrugated films are also useful in packaging applications either alone or when laminated to uncorrugated films such as described in assignees application of R- gers, application Ser. No. 113,849, filed Feb. 9, 1971, now US. Pat. No. 3,684,642. In such applications, the corrugated film contributes not only to the appearance and protection of the package but also provides dead fold properties which enable an opened package to be closed by creasing and also provides for the formation of sharp, well-defined corners on the package.

Corrugated films have a rather pronounced degree of stiffness in the direction of the corrugation lines. However, in a direction transverse to the corrugation lines, they exhibit less stiffness and even may tend to curl which may cause problems in film handling, especially on automatic film-handling equipment.

SUMMARY OF THE INVENTION It has been found that the stiffness of a corrugated polymeric film in a direction transverse to the direction of the lines of corrugation (hereinafter called TD stiffness") can be increased by flattening said film in a series of narrow width, noncorrugated lanes running in said transverse direction.

BRIEF DESCRIPTION OF THE DRAWING FIG. 11 is an isometric view of a portion of a typical film in accordance with this invention.

FIG. 2 is a portion of an enlarged sectional view of the film of FIG. 1 taken along cut 2-2.

DETAILED DESCRIPTION OF THE INVENTION The term corrugated is used herein to describe a film with a cross section such as that shown in FIG. 5 of Roberts, US. Pat. No. 3,244,799, having from about to about 40 corrugations per inch. The other geometrical configurations shown in that patent may also be used.

The term polymeric film includes regenerated cellulose films (cellophane) as well as films of polypropylene, polyesters, e.g., polyethylene terephthalate, highdensity polyethylene, and laminates thereof. The poly meric material should be rather high in modulus, i.e., above about 80,000 psi. The films of this invention before corrugation may range across a broad range of thicknesses; however, with films above about 5 mils, the need for stiffening in the direction transverse to the corrugation lanes is not as great. Accordingly, the stiffening afforded by the invention is most valuable on films between about 0.25 and 3.5 mils in thickness. The films may be oriented in one or both directions; however, if oriented, they must be heat set sufficiently to resist shrinkage during corrugation and during formation of the noncorrugated lanes.

The term noncorrugated lanes is used herein to mean and is illustrated in the drawing as a series of narrow width lanes 10 transverse to the direction of the corrugation lines 11 wherein the undulations of the corrugation have been substantially ironed fiat. The lane width A is not particularly critical but generally ranges from about one thirty-second to about one eighth of an inch. The frequency of the lanes can be varied to meet the particular stiffness properties desired in the film. It is usually desirable to have more than one but fewer than about eight noncorrugated lanes per inch. For most purposes, it is preferred to have between about two and six lanes per inch.

The film may be corrugated by running a preheated film between a pair of interfitting, meshing corrugating wheels which coact'to press the heated film into the desired longitudinally grooved, laterally waved, corrugated shape. The deeper the engagement of the grooves on the wheels, the greater the corrugation ratio obtained. The corrugation ratio is the ratio of the thickness of the corrugated film over the thickness of the uncorrugated film. If it is desired to have good dead fold in the corrugated film, the corrugation ratio should usually be at least about 6. The film is preheated between about 50 to 100C. by over and under contact with one or more of a series of internally heated rollers or drums usually about C. A preheating temperature should be high enough to avoid coating blush on coated films but not so high as to damage the coating. The preheated film is then led partially around and between the corrugating wheels which are both generally kept somewhere near room temperature by circulating tap water therethrough.

The stiffness in the transverse direction of the corrugated films is measured by the Gurley stiffness test, which, briefly described, comprises testing in a device which measures the resistance to bending, under conditions of uniform load, of a strip of film, e.g., 2 inches wide and 1 inch long, with the lanes running parallel to the length. The force in milligrams (mg.) is measured which is necessary to deflect the sample a given distance by application of the force near the end of the length of the strip.

Ordinarily, corrugated films of about 2.84 mils in thickness (before corrugation) have a TD stiffness of about 25 mg. which is too limp for many uses. However, a TD stiffness of over 100 mg. may be too stiff for many applications. It is generally desired in accordance with this invention to create a TD stiffness between about 50 to mg. As will be seen by one skilled in the art, this can be a function of film thickness, modulus, corrugation configuration and even corrugation ratio as well as the width and frequency of noncorrugated lanes created in the film transverse to the corrugation lines in accordance with this invention.

The narrow width, noncorrugated lanes may be formed transverse to the corrugation lines of the corrugated film in a number of ways which will readily occur to one skilled in the art. Generally speaking, they are best formed by a heated ironing out of the undulations in a defined path. A suitable method for forming the noncorrugated lanes will be described in connection with the following examples.

3 EX MP 1- A polyethylene terephthalate film laminate about 2.84 mils thick is prepared by laminating a pair of films 1.42 mils thick by heat and pressure and is corrugated as described hereinabove to a corrugation ratio of about 3.5 with about 32 corrugations per inch. The Gurley stiffness in a direction transverse to the corrugation lines is about 22 mg. The film is run into the nip formed by an embossing or ironing roll and a hard rubber-covered nip roll. The embossing roll is 16 inches long and 8.5 inches in diameter. One hundred seven grooves parallel to the roll axis are formed in the surface of the roll approximately three-sixteenths inch wide and three thirty-seconds inch deep leaving a onesixteenth inch uncut metal land between each groove. This will form four lanes per inch. The process variables are machine speed, nip roll pressure, and ironing roll temperature. The data appearing in the following Table are representative. In general, the stiffness is found to be related to roll temperature, nip roll pressure and film residence time on the embossing roll.

In experiments varying the numbers of lanes per inch, it is found that the corrugated film of the foregoing examples exhibits some increase in TD stiffness with even one lane per inch and a substantial increase with three lanes per inch.

It should be pointed out that creating the noncorrugated lanes in the film can have the effect of dropping the stiffness somewhat in the direction of the corrugation lines. However, since this stiffness is so high (e.g., about 400 mg.), a drop to about 300 mg. usually does not seriously affect the utility of the corrugated film.

It is surprising that the addition of the lanes improves the amount of TD stiffness to the degree noted. It is advantageous that the amount of flattening caused by the lanes does not adversely affect the dielectric strength of the film when used in electrical applications such as a cable core wrap. When the stiffened films of this invention are used to wrap cable cores, a more cylindricalcross section can be obtained than with unstiffened corrugated films.

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims, and all changes which come within the meaning and range of equivalence are intended to be embraced therein.

We claim:

1. In a corrugated polymeric film selected from the group consisting of polypropylene, thermoplastic polyesters, high-density polyethylene, and laminates thereof, having a thickness before corrugation of 0.25 to 3.5 mils, 20 to 40 corrugations per inch, a corrugation ratio of at least about 6, and a TD stiffness of less than about 38 mg. after corrugation, the improvement wherein said film has a series of noncorrugated lanes running transverse to the direction of the lines of corrugation, said lanes being about one thirty-second to oneeighth inch in width and there being two to six lanes per inch, such that the modified corrugated film has a TD stiffness between about 50 to mg. 

1. IN A CORRUGATED POLYMERIC FILM SELECTED FROM THE GROUP CONSISTING OF POLYPROPYLENE, THERMOPLASTIC POLYESTERS, HIGHDENSITY POLYETHYLENE, AND LAMINATES THEREOF, HAVING A THICKNESS BEFORE CORRUGATION OF 0.25 TO 3.5 MILS, 20 TO 40 CORRUGATIONS PER INCH, A CORRUGATION RATIO OF AT LEAST ABOUT 6, AND A TD STIFFNESS OF LESS THAN ABOUT 38 MG. AFTER CORRUGATION, THE IMPROVEMENT WHEREIN SAID FILM HAS A SERIES OF NONCORRUGATED LANES RUNNING TRANSVERSE TO THE DIRECTION OF THE LINES OF CORRUGATION, SAID LANES BEING ABOUT ONE THIRTY-SECOND TO ONE-EIGHT INCH IN WIDTH AND THERE BEING TWO TO SIX LANES PER INCH, SUCH THAT THE MODIFIED CORRUGATED FILM HAS A TD STIFFNESS BETWEEN ABOUT 50 TO 90 MG. 